a) Field of the Invention
The present invention relates to a radio communication system represented by a micro cellular system, and further to a channel allocation method suitable for the same.
b) Description of the Related Arts
A typical configuration of a micro cellular system is depicted in FIGS. 10 and 11 showing a cell layout and station-to-station interconnection, respectively.
In the cellular system, its coverage area is divided into a plurality of cells each being provided with a base station. Each cell is shaped like, for example, a hexagon as shown in FIG. 10, each base station taking charge of service within the associated cell where it is located. The main service each base station provides includes establishment and maintenance of a radio channel between a mobile station lying within the cell and the base station whereby a mobile station existing within a cell served by the base station can establish good communication with another mobile station or a fixed station (e.g., a station falling under another communication system). The mobile station herein represents a station installed in a vehicle or carried by a person. The radio channels for connecting a base station with a mobile station are referred to as a communication channel and a control information channel, and these are a channel for the communication between the two stations and a channel through which control information is transmitted for the establishment and maintenance of a communication channel, respectively.
Turning to FIG. 11, base stations 1 are individually arranged within each of the cells defined by an ellipse. The base stations 1 are each connected to a control station 5 by way of a control channel 4. The control channel 4 is in turn connected to other communication systems through, e.g., a public network. Each of the base stations 1, when a mobile station 2 lying within the cell it serves issues a call request via a control information channel (not shown), establishes a communication channel 3 between the mobile station 2 which has issued the call request and itself, and maintains this communication channel 3 until the completion of the communication. On the contrary, each of the base stations 1, when a mobile station 2 lying within the cell it serves is called by another mobile station 2 or a station constituting another communication system through another communication channel 3 or control channel 4, will establish a communication channel 3 between the mobile station 2 now being called and itself, and maintain this communication channel 3 until the communication is terminated. It is thus possible for the users of the mobile stations 2 to communicate with each other by way of the base station 1 and control station 5, and for the user of a mobile station 2 to communicate with the user of another communication system.
A plurality of communication channels 3 (or a plurality of frequencies) must be provided for due to the existence, normally, of a plurality of mobile stations 2, as well as interference waves in a single cell. Namely, in the case where a mobile station 2 within a cell uses a communication channel 3, another mobile station 2 lying within the same cell is not permitted to use the same communication channel 3. Further, a communication channel 3 unoccupied by another mobile station 2 but containing a significant interference wave would not ensure good communication. Therefore, the base station 1, when a mobile station 2 issues a call request, searches for an available communication channel (which is unoccupied by other mobile stations 2 and significantly uninfluenced by an interference wave) among a plurality of communication channels 3 provided for, and allocates the communication channel 3 found by the search to the mobile station 2 which has issued the call request. This processing is called channel allocation. Also, a cellular system including cells each having a reduced size of the order of 200 to 300 meters, is called a micro cellular system. The micro cellular system will allow a remarkable improvement in frequency resources consumption rate due to more effective reuse of the communication channels.
FIGS. 12 and 13 illustrate an exemplary configuration of the base station 1, and an action of its control circuit 7, respectively. The configuration and action are substantially disclosed in, e.g., "Autonomous Reuse Partitioning in MicroCellular Systems", Toshihiko KANAI, NEC corporation C&C Systems Research Laboratories, Technical Report of IEICE, CS91-32 (1991), pp. 23-29. FIG. 13 depicts, in particular, a flow of the channel allocation processing.
The base station 1 shown in FIG. 12 comprises a CIR (carrier to interference power ratio) measurement circuit 6, a control circuit 7, a channel search sequence memory 8 and a transmitter/receiver circuit 9. When a call request is issued from a mobile station 2, it is received by the transmitter/receiver circuit 9 by way of a control information channel. In response to the receipt of the call request, the control circuit 7 refers to the information contained on the channel search sequence memory 8 in the order of search sequence. Channel numbers specifying the communication channels 3 are contained in the channel search sequence memory 8. Accordingly, read into the control circuit 7 through a first reference after the receipt of the call request is a channel number corresponding to the search sequence=1, that is, #1 in FIG. 12. The control circuit 7 causes the transmitter/receiver circuit 9 to send, via a control information channel to the mobile station 2 which has issued a call request, a command to transmit a carrier through a communication channel 3 having a channel number #1 (100 in FIG. 13, hereinafter similarly referenced).
Upon receiving a command to transmit a carrier, the mobile station 2 transmits a carrier to the base station 1. Simultaneously or almost simultaneously with the transmission of the carrier from the mobile station 2, the control circuit 7 causes the transmitter/receiver circuit 9 to transmit a carrier to the mobile station 2. The CIR measurement circuit 6 measures a CIR of the carrier transmitted from the mobile station 2, while the mobile station 2 measures a CIR of the carrier transmitted from the base station 1. The CIR measured at the base station 1 represents a quality of the reverse link pertaining to the transmission from mobile station 2 to base station 1, of the communication channel 3 having a channel number #1, whereas the CIR measured at the mobile station 2 represents a quality of the forward link associated with the transmission from the base station 1 to mobile station 2. The CIR measurement circuit 6 supplies the thus measured CIR to the control circuit 7, and the mobile station 2 transmits the thus measured CIR via a control information transmission channel to the base station 1. The transmitter/receiver circuit 9 provides the received CIRs to the control circuit 7 (102).
The control circuit 7 functions to judge whether the CIR's at least a predetermined value (104). If at least one of the forward and reverse links is not judged to be at least the predetermined value, the control circuit 7 reads out the subsequent channel number sequentially contained in the channel search sequence memory 8, in this instance, #2. The control circuit 7 causes the transmitter/receiver circuit 9 to issue to the mobile station 2 a command to transmit a carrier through the communication 3 having a channel number #2 (106, 108).
When the CIR's of the forward and reverse links of the communication channel 3 having a channel number #2 are measured (102), the control circuit 7 Judges whether the CIR's are at least a predetermined value (104). If at some point after the repetition of such action the two CIR's are judged to be at least the predetermined value, then the control circuit 7 causes the transmitter/receiver circuit 9 to issue to the mobile station 2 a command to initiate a communication through the communication channel having such good CIR's (110). On the contrary, in the case of respective judgments of all the channel numbers stored in the channel search sequence memory 8 resulting in unsatisfactory CIR's, it will be considered to be impossible to execute channel allocation, thus leading to a call block (104, 106, 112).
One of the features of such a channel allocation method is that, among a plurality of communication channels, some communication channels having relatively small channel numbers are readily available. In other words, since the evaluation of CIR is conducted in the order of channel number, communication channels having relatively small channel numbers are frequently used in such a manner that #1 presents a highest frequency of use, #2 presents a second highest and so on.
A second feature is that some communication channels 3 having relatively small channel numbers are liable to undergo interference, which will lead to a deterioration of CIR. For instance, assume that two communication channels 3 having relatively small channel numbers are simultaneously used at some point. In this case, a carrier used in one of the two communication channels 3 is merely an interference wave when viewed from the other. Further, for instance, two communication channels 3 having close channel numbers are simultaneously used in the adjacent cells, respectively. A carrier wave used in one of the two communication channel 3 is also an interference wave when envisaged from the other. That is, as a result of a higher frequency of use of the communication channels 3 having relatively small channel numbers, some communication channels 3 having relatively small channel numbers tend to cause interference.
A third feature resides in the fact that the communication channels 3 having relatively small channel numbers tend to be allocated to the mobile stations 2 relatively closer to the base station 1, whereas the communication channels 3 having relatively large channel numbers tend to be allocated to the mobile stations 2 relatively distant from the base station 1. In other words, a shorter distance between a base station 1 and a mobile station 2 will reduce an attenuation of the carrier caused by radio transmission, which will in turn increase the carrier received power. On the contrary, a greater distance between a base station 1 and a mobile station 2 will increase an attenuation caused by radio transmission, which will in turn reduce the carrier received power. The CIR which is used as a judgment reference at the time of channel allocation is a ratio of the carrier received power to the interference wave received power, and hence a larger carrier wave received power will result in a good CIR value irrespective of a slightly larger interference wave received power. A simulation using as a model a system (refer to FIG. 10) including 36 (6 in a row.times.6 in a column) regular hexagonal cells resulted in a relationship between the channel numbers, and carrier received power and interference wave received power as shown in FIG. 14. Represented as "wanted wave" in this diagram is a carrier having a selected channel number. As is apparent from the result of this simulation, the communication channels having relatively smaller channel numbers are being used by mobile stations 2 relatively closer to the base stations 1 with a larger carrier received power.
These aspect of the third feature may entail a problem that the mobile stations 2 relatively distant from the base stations 1 are subjected to a larger connection delay. More specifically, upon allocating a communication channel 3 to relatively distant mobile stations 2 which will present a smaller carrier received power, repeatedly executed in the search order is a process including the steps of reading out a channel number 2 from the channel search sequence memory 8 and judging whether the CIR at that channel number is at least a predetermined value, which will increase the time taken to complete the channel allocating process. Additional time taken for the completion of the channel allocating process will lead to a need to provide the base stations 1 with a holding function allowing other mobile station 2 to stand by, so as to be capable of coping with the case when a call request is received from another mobile station 2 during the time which the channel allocation is being executed. In this manner, the conventional system involved problems such as poor service quality due to the connection delay, and complicated functions of the base stations.